I'll post some recommendations on how to upgrade that machine if you like, in the morning, but I want to get this up so you can consider it before I go to bed. You'd be a LOT better off getting rid of that machine and getting a newer G4 than you would be trying to upgrade that. You can resell that thing for what you paid for it (or close), and an AGP G4 is a MUCH more capable machine and not very much more expensive.
This one has 10hrs left... Seller has 100% positive feedback, no DOA guarantee that I see but it appears to be working. It's a very good deal with the amount of RAM it has in it. Your best upgrade right now would be to buy this machine:http://cgi.ebay.com/APPLE-G4-450MHZ-...QQcmdZViewItem